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Large carb meals, food absorption limits, delayed spikes

Here's a discovery that's been slowly crystallizing for a long time. I'm curious if anyone else has reproduced something like this:

If a meal is over 45g carbs, then only the first 45g absorbs in the bloodstream fast enough for the insulin to counteract it.

Bolusing over 45g all at once usually results in a low at about 2 hours. The bigger the meal, the more horrible the low. Two things are happening here - there's a larger surplus of insulin for a larger meal meaning a lower low, but also, the low is also harder to recover from, since glucose tabs are competing for digestion with the large meal.

3.5 hours later, the rest of the food appears to digest all at once, causing a monster spike. The bigger the meal over 45g, the bigger the spike is. (If the meal was a high-fat meal, the spike will be very difficult to bring down, but that's not the topic here.)

Combo boluses or square waves don't seem to work here. A square wave might still deliver too much up front. A combo bolus will frequently not give enough on the back end.

Correcting the spike at the 3.5 hour mark requires the insulin to be proportional to the food, not the blood sugar at that time. If the meal was large, then the correction factor is not enough to overwhelm the glucose entering the bloodstream. But the pump doesn't know this. It will try use the ISF to correct the BG, which won't work since ISFs assume the BG is not rising. Worse, it might still think you have some IOB and reduce the correction even further.

The food type doesn't seem to matter. 100g of pasta act similar to 100g of ice-cream, or 100g or chicken nuggets.

So here's what we do nowadays - when we are expecting a large meal, we bolus 45g carbs sight unseen. We'll even prebolus this at a restaurant after the order is in. (If food is very late, I know I can replace with juice or tabs or bread.) Then, when the meal comes in, we carb count everything, subtract the 45g, and write it down. For this example, let's say the whole meal was 100g. We eat in peace.

Then we watch the CGM. Like clockwork, at 3.5 hours the spike starts. Then, we bolus the remaining 55 carbs with no calcs or IOB. Bolus those carbs straight up. It's not perfect yet, but more than a few times we've avoided lows, and only stayed in the low 200's. Compare that to being stuck at 40 for 45 minutes and then 400 all night!

Do you notice this pattern? If so, do you find a different limit than 45g? Based on age or weight?

Yep, we see something similar, at about 60-70c for the total meal. We always prebolus 25-30 carbs, regardless of what he's eating. We'll bolus the rest of the 60-70c after the meal. If he eats more than that, I'll often keep an eye on the CGM and wait an hour or so to dose the rest of the carbs. The tricky part is that if I wait until the CGM is trending up, it's too late, and we deal with highs the rest of the night. I've got to catch it when it stops trending down, or is trending down slowly, and then have the intestinal fortitude to give him another dose, ignoring IOB and the fact that he's not yet going up. I agree that an extended dose doesn't really work for this. It's gotta be more than one "straight" bolus.

Yes, I've been thinking the same way. I even think, 'how many carbs are fast acting, how many are medium acting, and how many are long acting carbs?" and bolus/extend bolus accordingly. Funny how we've been at this for 4 years and my plan of attack is always evolving.....

Here's a discovery that's been slowly crystallizing for a long time. I'm curious if anyone else has reproduced something like this:

If a meal is over 45g carbs, then only the first 45g absorbs in the bloodstream fast enough for the insulin to counteract it.

Bolusing over 45g all at once usually results in a low at about 2 hours. The bigger the meal, the more horrible the low. Two things are happening here - there's a larger surplus of insulin for a larger meal meaning a lower low, but also, the low is also harder to recover from, since glucose tabs are competing for digestion with the large meal.

3.5 hours later, the rest of the food appears to digest all at once, causing a monster spike. The bigger the meal over 45g, the bigger the spike is. (If the meal was a high-fat meal, the spike will be very difficult to bring down, but that's not the topic here.)

Combo boluses or square waves don't seem to work here. A square wave might still deliver too much up front. A combo bolus will frequently not give enough on the back end.

Correcting the spike at the 3.5 hour mark requires the insulin to be proportional to the food, not the blood sugar at that time. If the meal was large, then the correction factor is not enough to overwhelm the glucose entering the bloodstream. But the pump doesn't know this. It will try use the ISF to correct the BG, which won't work since ISFs assume the BG is not rising. Worse, it might still think you have some IOB and reduce the correction even further.

The food type doesn't seem to matter. 100g of pasta act similar to 100g of ice-cream, or 100g or chicken nuggets.

So here's what we do nowadays - when we are expecting a large meal, we bolus 45g carbs sight unseen. We'll even prebolus this at a restaurant after the order is in. (If food is very late, I know I can replace with juice or tabs or bread.) Then, when the meal comes in, we carb count everything, subtract the 45g, and write it down. For this example, let's say the whole meal was 100g. We eat in peace.

Then we watch the CGM. Like clockwork, at 3.5 hours the spike starts. Then, we bolus the remaining 55 carbs with no calcs or IOB. Bolus those carbs straight up. It's not perfect yet, but more than a few times we've avoided lows, and only stayed in the low 200's. Compare that to being stuck at 40 for 45 minutes and then 400 all night!

Do you notice this pattern? If so, do you find a different limit than 45g? Based on age or weight?

We are on MDI, but we don't see this with all foods. We tend to see it with pasta (so far anyway) - DD eats about 70 g carbs practically with every meal, and we don't usually end up with numbers increasing after, although we do notice the lows within an hour or so after the shot.

We are on MDI, but we don't see this with all foods. We tend to see it with pasta (so far anyway) - DD eats about 70 g carbs practically with every meal, and we don't usually end up with numbers increasing after, although we do notice the lows within an hour or so after the shot.

Interesting. I think the limit might be somewhat larger if the child is bigger. Also, you're early in DX. We didn't see this behavior until well after a year into dx when the honeymoon ended. Pizza used to be easy at one time... I miss those days!

We have definitely had this problem. For us it's usually around 80-90 carbs but we typically don't do anything different until it's over 100 carbs. Since we are MDI what we do to combat this is give 1/2 of the bolus up front and then about a hour after eating we re-test to make sure he's not too low (we don't have a CGM) and then we give the other 1/2 of the bolus. It's worked pretty good for us but sometime that hour wait is too long and sometimes it's too short. I was hoping that when we start the pump in two weeks that the combo bolus/square wave/temp basal's would fix this but it sounds like you still have the same problem.

I figured I'd update a little bit on this. We've been pushing the limits a bit more and trying to figure out higher-fat food like pizza. Last Friday we got up to 150g of pizza.

The 40-carb rule seems to hold, so no more than 40 up front, otherwise we get a low.

However, sometimes bolusing all the rest (say, 100g when the spike starts) doesn't work. There seems to be a practical limit to how big a bolus can be, before it just sort of disappears into the skin, not to mention dealing with fat spikes all night.

So, what we are doing now is this.

1. Bolus 40g up front.
2. Wait for CGM to spike. When it does, bolus another 40g.
3. If the BG responds to the second 40g, then wait for another spike, and keep repeating 40g increments until the meal is done.
4. If it doesn't, then this is the Big Spike, and we go into "pizza mode". Bolus the uncovered part of the meal (say, 70g) but extend it over 30m-1h so it absorbs better.
5. Then, to deal with the fat spike, start a 200-210% temp basal for 8-12 hours. (Not +200, it's just over double.) That's right, 200% appears to be the minimum, at least for my son.

What am I taking away from this? Everyone's different, so there's a ton of problem solving to do. I hope this gives you some ideas on what to try, and how different the response can be.

Doing it without a CGM is nearly impossible, too... I can't imagine taking a finger stick every 20 minutes to see where everything lines up.

The only twist I'd add is that we can never wait for a spike to begin rising to address it, no matter why. Too slow for us, that insulin.

This thread reminds me of something I still have a hard time wrapping my brain around -- the idea that injecting more insulin doesn't make it work any faster. I always tend to want to add more insulin to get it going quicker, and have to remind myself that's not the way it works. Timing and dose have to be thought about and calculated separately.

Interesting. I find that if the CGM trend does not change withing 15-20 minutes, then the dose is probably wrong. That means a double-arrow up should at least go to angle-up, or a level arrow should give one arrow down.

(For those of you not using a Dex, there are 7 trends:

Two arrows up
One arrow up
One diagonal arrow up ("angled up")
Horizontal arrow across ("straight arrow")
One diagonal arrow down ("angled down")
One arrow down
Two arrows down

From my observations, I'm saying that a bolus, should fairly quickly bring you 2-3 trends levels downward. If not, either the basal or bolus isn't going to work.)